1. Field of the Invention
The present invention relates to an optical element used in optical devices such as video cameras, digital cameras, still video cameras, copiers, and so on and, more particularly, to an optical element with an imaging action having a structure of a plurality of curved, reflecting surfaces.
2. Related Background Art
Various proposals have been made heretofore about mirror optical systems making use of reflecting surfaces such as concave mirrors, convex mirrors, and so on. As an example of mirror optical systems, there is the so-called Cassegrain reflector telescope constructed for the purpose of decreasing the entire length of the optical system by folding the optical paths of the telephotographic lens system of the long entire lens length consisting of refracting lenses, by use of two opposed, reflecting mirrors.
For the objective system of telescopes, there are a number of known types to decrease the entire length of the optical system by use of a plurality of reflecting mirrors, in addition to the Cassegrain type, for the same reason.
In this way the compact mirror optical systems have been constructed heretofore by efficiently folding the optical paths, using the reflecting mirrors in place of lenses in the taking lens unit of the long entire lens length.
In these reflection type photographing optical systems, optical components need to be assembled with accuracy in order to achieve desired optical performance. Particularly, since errors in relative position accuracy of the reflecting mirrors strongly affect the optical performance, it is important to accurately adjust the position and angle of each reflecting mirror.
A method proposed as a solution to this problem is a method of constructing a mirror system with a plurality of reflectors from one block, thereby avoiding the assembly errors of the optical components during assembly.
For example, as to non-coaxial optical systems, it is known that the optical systems with well-corrected aberration can be constructed by introducing the conception of a reference axis and forming the constituent surfaces of asymmetric, aspherical surfaces; Japanese Patent Application Laid-Open No. 9-5650 describes the designing method thereof, and Japanese Patent Applications Laid-Open No. 8-292371, No. 8-292372, No. 9-222561, and No. 9-258105 describe the design examples thereof.
Such non-coaxial optical systems are called off-axial optical systems (which are optical systems defined as optical systems including a curved surface (off-axial curved surface) a normal to which at an intersection between the reference axis and the constituent surface is not present on the reference axis, the reference axis being an axis along a ray passing the center of the image (or the center of the object) and the center of the pupil, wherein the reference axis is bent).
In these off-axial optical systems, each constituent surface is generally non-coaxial and no eclipse will occur even if it is a reflecting surface; therefore, it is easy to construct the optical system with reflecting surfaces. They also have such features that routing of optical paths is relatively free and that it is easy to make an integral optical system by an integral molding technique of constituent surfaces.
FIG. 15 is a schematic diagram to show an embodiment of the reflecting optical system disclosed in Japanese Patent Application Laid-Open No. 8-292371.
In FIG. 15, numeral 21 designates an optical element having a plurality of curved, reflecting surfaces, which is made of a transparent body of glass or the like.
In the same drawing, light from an object OB passes a stop 1 and enters the reflection type optical element 21. In the optical element 21 the light is refracted at a first surface R1, is reflected at a second surface R2, a third surface R3, a fourth surface R4, a fifth surface R5, and a sixth surface R6, is refracted at a seventh surface R7, and then emerges from the optical element 21. At this time, the light forms a primary image on an intermediate image plane near the second surface R2 and forms a pupil near the fifth surface R5. Then the light emerging from the optical element 21 finally forms an image on an image pickup surface (an image pickup surface of an image pickup medium such as CCD or the like) 4.
In the prior art example of FIG. 15, the mirror optical system is constructed using the optical element 21 in which the reflecting surfaces consisting of a plurality of curved surfaces and/or planes are integrally formed, whereby the optical system can be constructed in a compact overall structure and with relaxed arrangement accuracy (assembly accuracy) of the reflecting mirrors, which is otherwise often high in the mirror optical system.
Further, the stop 1 is placed on the object side of the optical element 21 and the object image is formed at least once in the optical element, whereby the effective diameter of the optical element is reduced in spite of the construction of the reflection type optical element with a wide angle of view. In addition, a proper optical power is given to the plurality of reflecting surfaces forming the optical element and each reflecting surface of the optical element 21 is decentered, whereby the optical path in the optical element is bent in the desired shape, so as to decrease the entire length in a predetermined direction of the optical element.
The role of each surface will be described with reference to FIG. 16 for the reflective optical element with five off-axial reflecting surfaces as illustrated in FIG. 15. Let us define the entrance refracting surface as R1, the off-axial reflecting surfaces as R2 to R6, and the exit refracting surface as R7 in the order of passage of the light incident to the reflective optical element 21. Then a first component B1 is defined from the entrance refracting surface R1 to the off-axial reflecting surface R2, a second component B2 is defined from the off-axial reflecting surface R3 to the off-axial reflecting surface R5, and a third component B3 is defined from the off-axial reflecting surface R6 to the exit refracting surface. This means that the reflective optical element 21 consisting of the five off-axial reflecting surfaces is replaced by the three optical components. The first component B1 plays the role of intermediately imaging the incident light from the object, the second component B2 plays the role of focusing the light from the intermediate image plane at the pupil, and the third component B3 plays the role of secondarily focusing the light from the pupil image plane at the image pickup surface.
FIG. 17 shows the state of distortion and FIG. 18 the state of aberration on the image plane, of the reflective optical element illustrated in FIG. 15. As seen from FIG. 17 and FIG. 18, the distortion and aberration increases with distance from the center of the image plane.
When this embodiment is divided into three components of the first component B1 to the third component B3 of the reflective optical element as illustrated in FIG. 16, the first component B1 to the third component B3 all have their respective, positive focal lengths (positive optical powers).
In general, in the case of the optical element illustrated in FIG. 15, if the size of the intermediate image plane is large, i.e., if the focal length of the first component B1 is long in FIG. 16, effective diameters of the reflecting surfaces after the intermediate image plane will become large and it will result in making it difficult to decrease the size of the optical element. If the size of the intermediate image plane is too small, i.e., if the focal length of the first component B1 is too short, it will be difficult to correct the aberration by the optical surfaces after the intermediate image plane.
An object of the present invention is to provide a reflective optical element with a wide angle of view and with a reduced effective diameter, by properly arranging the power layout of surfaces so as to achieve excellent optical performance and by setting the focal length from the entrance surface to the intermediate image to an appropriate value, where the object image is formed on a predetermined plane by use of the optical element in which a plurality of reflecting surfaces consisting of curved surfaces and/or planes are formed on surfaces of a transparent body.
For accomplishing the above object, an optical system of the present invention is an optical system for forming an image of an object with light from the object, comprising:
an aperture stop; and
an optical unit placed on the image side of the aperture stop, the optical unit comprising the following components in the order named from the object side:
a first optical component with a reflective curved surface, for forming an intermediate image of the object,
a second optical component with a reflective curved surface, for forming an image of the aperture stop with light from the intermediate image of the object, and
a third optical component with a reflective curved surface, for forming a secondary image of the object with light from the image of the aperture stop;
wherein, where fB1(xcex8), fB2(xcex8), and fB3(xcex8) are focal lengths of the first optical component, the second optical component, and the third optical component, respectively, and f(xcex8) is a total focal length of the optical unit, at an azimuth xcex8, the focal lengths satisfy the following condition A or the following condition B in the azimuth range of 0 less than xcex8 less than 2xcfx80:
Condition A:
fB1(xcex8) greater than 0, fB2(xcex8) less than 0 and fB3(xcex8) greater than 0
Condition B:
fB1(xcex8) greater than 0, fB2(xcex8) greater than 10|f(xcex8)| and
fB3(xcex8) greater than 0.
An optical element according to one aspect of the present invention is an optical element comprising:
a plurality of reflective curved surfaces for successively reflecting light incident on the optical element, the plurality of reflective curved surfaces comprising a reflective curved surface for first reflecting the light incident on the optical element, as a surface A, a reflective curved surface for next reflecting the light reflected by the surface A, as a surface B, a reflective curved surface for finally reflecting the light incident on the optical element, as a surface D, and a reflective curved surface immediately before the surface D, as a surface C,
wherein, in an order of path of the light incident to the optical element, a first optical component is defined before the surface A, a second optical component is defined from the surface B to the surface C, and a third optical component is defined after the surface D and wherein, where fB1(xcex8), fB2(xcex8), and fB3(xcex8) are focal lengths of the first optical component, the second optical component, and the third optical component, respectively, and f(xcex8) is a total focal length of the optical element, at an azimuth xcex8, and the focal lengths satisfy the following condition A or the following condition B in the azimuth range of 0 less than xcex8 less than 2xcfx80:
Condition A:
fB1(xcex8) greater than 0, fB2(xcex8) less than 0 and fB3(xcex8) greater than 0
Condition B:
fB1(xcex8) greater than 0, fB2(xcex8) greater than 10|f(xcex8)| and
fB3(xcex8) greater than 0.
Each of optical devices according to the present invention comprises the optical system or either of the optical elements of the present invention as described above.